A composite color video signal includes horizontal synchronizing signals, a burst signal superimposed onto the pedestal level at the back porch of the horizontal synchronizing signal and a video information signal. The video information signal comprises a chrominance subcarrier having different phases amplitude-modulated with chrominance information. The composite color video signal includes both luminance and chrominance information. The burst signal has a burst signal frequency equal to 3.58 MHz, which is the frequency of the chrominance subcarrier f.sub.sc. The video signal can also be separated into a separate chrominance or C signal and separate luminance or Y signal. A mixer circuit can be used to combine the separate chrominance or C signal with the separate luminance or Y signal to thereby form a composite video signal.
In video circuits, it is sometimes necessary to implement a trap, or filter, to remove unwanted frequencies from the luminance signal which would interfere with the chrominance signal to prevent problems in a video signal such as cross coloring. Previously, trap filters have been implemented using external components, such as an inductor and capacitor (LC) filter, coupled to a pin of an integrated circuit. The separate luminance signal is then routed to flow through the pin and is filtered by the external LC filter, thereby allowing circuit and system designers the ability to implement the filter with precision external components appropriate for specific systems and desired values. Such an external LC trap filter is illustrated in FIG. 1. The separate luminance signal Y is coupled internally to a pin 10. The pin 10 is coupled to a mixer 16 which also receives the separate chrominance signal C. The mixer 16 combines the separate luminance signal Y and the separate chrominance signal C, thereby forming a composite video signal CV. To filter the unwanted frequencies from the separate luminance signal Y before it is combined with the separate chrominance signal C, an external trap filter 18 is coupled to the pin 10. The external trap filter 18 includes a capacitor 12 and an inductor 14 which together form an LC filter used to filter the unwanted frequencies from the separate luminance signal Y.
The implementation of such an external trap filter including an external capacitor and an external inductor is relatively expensive due to the cost of the required external components. The external components also take up valuable space on a printed circuit board or within a system. What is needed is an internal trap filter which can be selectively engaged and does not require an external inductor and capacitor. What is further needed is a circuit which allows a single pin to be used to selectively access an internal trap filter or to implement an external trap filter.